Wireless communication systems, for example cellular telephony or private mobile radio (PMR) communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs). The term mobile station generally includes both hand-portable and vehicular mounted radio units. Radio frequency (RF) transmitters are located in both BTSs and MSs in order to facilitate wireless communication between the communication units.
In the related technical field, it is known that there is continuing pressure on a limited radio spectrum available for radio communication systems, which is focusing attention on a development of spectrally efficient linear modulation schemes. By using spectrally efficient linear modulation schemes, more communication units are able to share allocated spectrum within a defined geographical coverage area (communication cell). An example of a digital mobile radio system that uses a linear modulation method, such as π/4 digital quaternary phase shift keying (DQPSK), is a TErrestrial Trunked RAdio (TETRA) system, developed by the European Telecommunications Standards Institute (ETSI).
Since envelopes of these linear modulation schemes fluctuate, intermodulation products can be generated in non-linear RF power amplifier(s) (PAs). Specifically in the digital PMR market, restrictions on out-of-band (interfering) emissions are severe (to an order of −60 dBc to −70 dBc relative to a power in adjacent frequency channels). Hence, linear modulation schemes used in this scenario require highly linear transmitters.
An actual level of linearity needed to meet particular out-of-band emission limits, is a function of many parameters, of which the most critical parameters are modulation type and bit rate. Quantum processes within a typical RF PA device are non-linear by nature. A straight line may only approximate a transfer function of the power amplifier when a small portion of consumed direct current (DC) power is transformed into RF power, as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable units.
One emphasis in portable PMR equipment is to increase battery life. Hence, it is useful to maximise operating efficiencies of the amplifiers used. To achieve both linearity and efficiency, so called linearisation techniques are used to improve a linearity performance of the more efficient classes of amplifier, for example class AB, B or C amplifiers. One such linearisation technique, often used in designing linear transmitters, is Cartesian Feedback. This is a ‘closed loop’ negative feedback technique, which sums a baseband feedback signal in its digital ‘I’ and ‘Q’ formats with a corresponding generated ‘I’ and ‘Q’ input signals in a forward path. Linearising of the PA output requires accurate setting and on-going control of a phase and amplitude of a feedback signal. Details of an operation of such a linearisation technique are described in the paper “Transmitter Linearisation using Cartesian Feedback for Linear (time division multiple access) TDMA Modulation” by M. Johansson and T. Mattsson 1991 IEEE.
A lineariser circuit optimises a performance of a transmitter, for example to comply with linearity or output power specifications of a communication system, or to optimise an operating efficiency of the transmitter power amplifier. Operational parameters of the transmitter are adjusted to optimise the transmitter performance and include, as an example, one or more of the following: amplifier bias voltage level, input power level, phase shift of a signal around a feedback path. Such adjustments are performed by say, a microprocessor. Due to a sensitivity performance of such transmitter circuits, a range of control and adjustment circuits and/or components are needed so that a linear and stable output signal can be achieved under all operating circumstances.
All linearisation techniques require a finite amount of time in which to linearise the performance of a given amplifying device. The ‘linearisation’ of the amplifying device is often achieved by initially applying a training sequence to a lineariser circuit and the amplifying device in order to determine levels of phase and gain distortion introduced by the linearisation loop and the amplifying device. Once phase and gain distortion levels have been determined, they can be compensated for, generally by adjusting feedback components/parameters.
To accommodate for such linearisation requirements, communication systems typically allocate specific training periods for individual users to train their transmitters. The TErrestrial Trunked RAdio (TETRA) standard includes a time frame, termed a Common Linearisation Channel (CLCH) as is described in UK Patent Application No. 9222922.8, to provide a full-training period approximately once every second. The CLCH frame allows a radio to ‘train’ prior to gaining access to the TETRA communication system. However, a radio having to wait up to one second before training and then accessing the system is undesirable. To minimise the effect of this significant delay in call set-up times, and also provide an additional period for fine tuning a radio's output characteristics, due to changes in temperature, supply voltage or frequency of operation, a reduced training sequence has been inserted at the beginning of each TETRA traffic time slot for a radio allocated that slot to perform a minimal amount of training or fine tuning. This period may be used for phase training.
An example of such a training sequence is described in U.S. Pat. No. 5,066,923 of Motorola Inc., which describes a training scheme where a phase of a transmitter amplifier is adjusted in an ‘open-loop’ mode and a gain of the transmitter amplifier is adjusted when a feedback loop is closed.
During phase training, a Cartesian feedback loop may be configured to be ‘open loop’, for example, a switch may be used to prevent the fed-back signal from being combined with a signal routed through the transmitter.
FIG. 1 illustrates a phase diagram 100 with a perfect I/Q quadrature balance, namely a 90-degree phase difference between the ‘I’-channel 120 and the ‘Q’-channel 110. One known method for controlling/setting phase and amplitude levels around the feedback loop is described here. The Cartesian loop is opened and a positive baseband signal applied to an input of the ‘I’-channel. Phase training control circuitry monitors a signal before switching on the ‘Q’-channel—indicated as Vfq 140. A successive approximation register (SAR) phase training algorithm controls a phase shifter and is arranged to minimise the Vfq voltage. Once the SAR algorithm has completed, a phase correction signal corrects a loop phase from Vfq 140 to Vfq_t 130 by an angle □ 150. A voltage value measured on the ‘Q’-channel prior to the switch may then be reduced to a level close to zero. A same process may be repeated for a negative baseband signal input to the ‘I’-channel. Calculated results from both positive and negative training applied to the ‘I’-channel are averaged and used to adjust the phase shift around both the ‘I’-channel loop and the ‘Q’-channel loop.
In practice, a perfect I-Q 90-degree relationship is rarely achieved. This imbalance results from various component tolerances within the respective ‘I’ and ‘Q’ loops.
It is known that polyphase quadrature generators, which are commonly used to generate I-Q signals, are inherently narrowband in nature. Therefore, in order to cover frequency bands of 100-1000 MHz, or possibly 100 MHz-5 GHz, many polyphase quadrature generators are needed in known frequency generator circuits to cover the desired bandwidth. Alternatively, it is possible to use a number of frequency doublers and divide-by-2 quadrature generators. This also provides a wideband solution. However, employing such an approach may create two problems that may need to be addressed:
(i) High noise from frequency doublers; and
(ii) IQ phase ambiguity during phase training.
It is also known that phase adjustments are performed on a downmixer during a transmit time slot, as described, for example in U.S. Pat. No. 6,731,694, in order to remove a need for a costly and large circulator. However, such phase adjustments have been found to cause adjacent channel interference (sometimes referred to as ‘splatter’).
Thus, there currently exists a need to provide an improved wireless communication unit, a transmitter linearisation integrated circuit, and an improved method of linearising a transmitter in a wireless communication unit, wherein the abovementioned disadvantages may be alleviated.